U.S. patent number 3,836,156 [Application Number 05/173,321] was granted by the patent office on 1974-09-17 for ablative seal.
This patent grant is currently assigned to United Aircraft of Canada Limited. Invention is credited to Hector B. Dunthorne.
United States Patent |
3,836,156 |
Dunthorne |
September 17, 1974 |
ABLATIVE SEAL
Abstract
A composite sealing structure for sealing the gap between
stationary and moving surfaces of a gas impelling device, for
example a turbine. Preferably, the sealing structure is on the
surface of the stationary member. The sealing structure is adapted,
in the event of a rub, to melt immediately at the rubbed surface
whereby the metal is molten and readily displaced to allow free
passage of the rubbing surface of the other member. The sealing
structure includes an outer sealing layer as the proximate surface
of one member and an intermediate bonding layer bonding the sealing
layer to the surface of the member. The sealing layer is of a metal
alloy having a melting point close to, but above the operating
temperature in the impelling device and below the melting point of
the surface of the one member. The bonding layer is of an alloy
compatible with the sealing layer alloy and with the base metal,
forms a firm bond with both the sealing layer and the base metal
surface, is adapted to melt initially at a temperature lower than
that of either, has a remelt temperature above the operating
temperature of the impelling device, and good strength at the
operating temperature. Preferably, the alloys used for the sealing
and bonding layers are brazing alloys.
Inventors: |
Dunthorne; Hector B. (Verdun,
Quebec, CA) |
Assignee: |
United Aircraft of Canada
Limited (Longueuil, Quebec, CA)
|
Family
ID: |
10356269 |
Appl.
No.: |
05/173,321 |
Filed: |
August 19, 1971 |
Foreign Application Priority Data
|
|
|
|
|
Jul 19, 1971 [GB] |
|
|
33696/71 |
|
Current U.S.
Class: |
277/415;
415/173.4; 415/174.4; 277/933; 277/940 |
Current CPC
Class: |
F01D
11/12 (20130101); F16J 15/445 (20130101); Y10S
277/933 (20130101); Y10S 277/94 (20130101) |
Current International
Class: |
F16J
15/44 (20060101); F01D 11/12 (20060101); F01D
11/08 (20060101); F16j 015/44 (); F01d
011/08 () |
Field of
Search: |
;277/26,53
;415/9,174 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Prince; Louis R.
Assistant Examiner: Smith; Robert I.
Attorney, Agent or Firm: Swabey; Alan
Claims
I claim:
1. Sealing means for a gas impelling device including surfaces in
close proximity between which there is relative movement, one
surface being that of a stationary member and the other surface
being that of a rotating member, comprising,
a composite sealing structure including an outer sealing layer
constituting the proximate surface of one member and an
intermediate bonding layer bonding the sealing layer to the base
surface of said one member,
the sealing layer being of a metal alloy such that its melting
point is close to but above the operating temperature in the
impelling device and below the melting point of the other
member,
the bonding layer being of an alloy compatible with the alloy of
the sealing layer and with the base metal and forming a firm bond
with both the sealing layer and the base metal surface and being
adapted to melt initially at a temperature lower than the
temperature of either and having a remelt temperature above the
operating temperature of the impelling device, and having good
strength at said operating temperature,
the sealing structure being adapted, in the event of a rub, to melt
immediately at the rubbed surface whereby the metal is molten and
readily displaced to allow free passage of the rubbing surface of
the other member.
2. Sealing means, as defined in claim 1, in which the sealing
structure is on the surface of the stationary member.
3. Sealing means, as defined in claim 2, in which the gas impelling
device is a gas turbine.
4. Sealing means, as defined in claim 1, in which the alloys used
for the sealing layer and the bonding layer are brazing alloys.
5. Sealing means as defined in claim 1, in which the sealing layer
consists of an alloy having the following composition:
Cr: 14.0
B : 3.5
si: 4.5
Fe: 4.5
C : 0.7
ni: balance the parts being in per cent by weight.
6. Sealing means as defined in claim 5, in which the bonding layer
consists of an alloy having the following composition:
Cr: 15.0
Fe: 7.0
Si: 0.75
B : 0.2
ni: balance the parts being in per cent by weight.
Description
BACKGROUND OF THE INVENTION
This invention relates to sealing the gap between the rotating and
stationary members of a gas impelling device.
In many cases, the efficiency of gas impelling devices, for
example, gas turbines and compressors, depends on minimizing this
gap. Because of the differences in the co-efficient of expansion of
the respective metal components running close together within
narrow tolerances, as the temperature rises in the device, it is
necessary either to leave a large enough gap to allow for the
expansion so as to avoid rubbing at all extremes of operating
temperature or to provide for limited rubbing between the rotating
and stationary members by providing means to prevent damage to the
rotating or stationary member or both.
This has given rise to various attempts to provide a seal between
the stationary and rotating parts of gas impelling devices. One of
these attempts is to use an abradable surfacing material on one of
the rubbing parts. The abradability is achieved by reducing the
density by low sintering pressures applied to metallic powder or
fibers in forming the surfacing material, or by the inclusion in
the surfacing material of friable non-metallic materials as for
example graphite or diatomaceous earth. Such seals are expensive to
produce. Sintered or sprayed layers also have poor adhesion to the
base material and their open texture permits oxidation and
consequent embrittlement. They are also susceptible to rapid
erosion by engine carbon and dust particles. Moreover, increasing
the density of the layer to overcome these problems can result in a
resistant surface which damages the rotating part.
Attempts have also been made to surface the parts subject to
rubbing contact with a material which fuses at the temperature
induced by the friction of a rub. In this connection, the use has
been suggested of eutectic alloys with fixed melting points
(similar to a pure metal) and alloys with a "mushy" melting
condition. A eutectic alloy would be ideal in that the minimum
quantity of heat would be needed to take the seal from solid to
liquid at the point or line of contact with the knife edge.
However, there are other important considerations. An alloy with a
larger range between the solidus temperature (first sign of
melting) and the liquidus temperature (at which the entire alloy is
melted) may have a lower thermal conductivity which would tend to
concentrate the heat at the point or line of contact. The "mushy"
alloy will melt in the films between the higher melting point
constituents which destroys its effective strength so that the
material is displaced with less total heat input. The
"compressible" type of seal relies on deforming the material and
the "abradable" type of seal relies on chipping or gouging the
material. Both types of seals suffer the limitation that the very
high temperature generated at the line of contact may be sufficient
to flow the surface material so that when the rubbing ceases a hard
skin is formed which could damage the rotating member at the next
contact.
Such materials are difficult to attach securely to the base
material and are also degraded by the abrasive and erosive action
of solid particles in the gas stream.
SUMMARY OF THE INVENTION
The present invention aims to overcome the shortcomings of these
prior expedients as well as to provide certain other advantages as
will be clear from the following. The applicant provides a sealing
structure such that in the event of a rub between the rotating and
stationary members, the seal construction melts instantaneously at
the point of contact to allow the metal to be displaced, permitting
free rotation of the rotatable member. This is provided according
to the invention as follows. Sealing means is applied to one of the
surfaces of the seal, usually to the stationary surface, including
a sealing layer of a metal alloy having a melting point close to,
but above, the operating temperature of the rotating surface and
below the melting point of the stationary surface. An intermediate
bonding layer bonds the sealing layer to the surface. The bonding
layer is of an alloy compatible with the sealing layer and with the
metal of the base surface such that it will form a good brazed bond
with both and such that it will melt initially at a temperature
lower than the temperature of either and will have a remelt
temperature above said operating temperature and adequate strength
at the operating temperature. The material of the sealing layer is
adapted, in the event of a rub to melt instantaneously at the
contact surface in the immediate area of the rub whereby the metal
is molten and is readily displaced to allow free rotation of the
rotatable member. Unlike seals, in which the seal material is
merely abraded or deformed, the sealing layer is of a material
which is fully dense or of reduced controlled density.
In accordance with the invention the sealing layer can be applied
by conventional metal application techniques, for example such as
those used in applying the brazing alloys contained in a vehicle
and subsequent fusion. These techniques include spraying or
painting on a paste with the alloy in finely divided form in a
vehicle, which may, for instance be a synthetic resin, or the alloy
may be contained in a solid metal strip attachable to the substrate
by brazing.
More specifically, characteristics of the alloy of the sealing
layer are as follows. It must be oxidation resistant. It must not
flow when being fused to the bonding layer. It must be such at
operating temperature the seal does not sag or run. The face of the
seal will normally be solid or fully dense in character without
significant porosity, but reduced density in the form of discrete
voids is permissible up to the limits imposed by the reduction of
mechanical strength, erosion and corrosion resistance or sealing
efficiency which is up to about 30 percent reduction in density.
Reduced density would increase leakage past the knife edges and
more rapid oxidation under the high temperature application. This
contrasts with the compressible type of seal which is dependent on
low density.
The invention will be further explained by reference to the
accompanying drawing illustrating a preferred embodiment.
FIG. 1 is a fragmentary cross-section showing a stationary shroud
over a stator vane and a rotor vane; and
FIG. 2 is an enlarged fragmentary view of the seal bonded to the
shroud.
The following is a typical application of the present invention in
providing a seal between the stationary shroud 6 and a rotating
impeller vane 8 of turbines. This seal 10 was applied to the second
and third stage turbine shrouds of the engine known under the model
number JT15D. The bonding layer 12 was of "Nicrobraz 125" (trade
mark) representing an alloy having the following composition:
Cr : 14.0
B : 3.5
si : 4.5
Fe : 4.5
C : 0.7
ni : balance and "Nicrogap 108" (trade mark) representing an alloy
have the following composition:
Cr: 15.0
Fe: 7.0
Si: 0.75
B : 0.2
ni: balance for the sealing layer these alloys are disclosed in
"Nicrobraz Engineering Data Sheet No. 2.1.2 and 2.2.14"
respectively published by Wall Colmonoy Corp. in 1966. "Nicrobraz
125" has a brazing range of 1,950.degree.F to 2,200.degree.F.
"Nicrogap 108" has a melting point above 2,400.degree.F, but is
diluted by the "Nicrobraz 125" so that the remelt temperature was
above 2,000.degree.F. The tip speed on the second and third stage
turbine blades was approximately 1,000 ft./sec. and the
temperatures about 1,300.degree.F and 1,100.degree.F respectively.
The tip clearance was nominally 0.015 inches. The thickness of the
bonding layer was about 0.005 inches and either 0.010 inches or
0.040 inches for the face layer depending on the amount of gap to
be filled on the particular engine. The "Nicrobraz 125" was sprayed
on and air dried. It was then fused in hydrogen. The "Nicrogap 108"
was sprayed over the fused "Nicrobraz 125" and air dried. Excess
alloy was scraped off. The seal was then fused.
As shown in the drawing, the shroud 6 includes an annular recess
into which the seal 10 is provided and held to the shroud by means
of the bonding layer 12.
* * * * *